Intermediate in the synthesis of folic acids. Prepn: Waller et al., J. Am. Chem. Soc. 70, 19 (1948); cf. Hultquist et al., ibid. 23; Boothe, US 2472462 (1949 to Am. Cyanamid); C. Temple et al., J. Org. Chem. 46, 3666 (1981). Synthesis: M. G. Nair et al., ibid. 3152.